The invention is applicable to aircraft construction, for example.
Document FR-2 873 315 describes a positive feed drill, i.e. a drilling machine for drilling a bore or passage, having a drive mechanism, and being of the kind known as a positive feed drill. A single motor drives the spindle in rotation about its axis, via the drive mechanism, while at the same time advancing or reversing the spindle in translation along its axis.
Since the drives in translation and in rotation for the spindle are linked mechanically together, the amount by which the spindle is advanced on every revolution, that is to say the amplitude of the movement in translation of the spindle for each revolution of the spindle, is constant. Variations in motor speed do not therefore affect the magnitude of this advance per revolution. The thickness of the swarf that is formed therefore stays constant, which is beneficial in terms of surface quality and precision of the bores (or passages) drilled by such a machine tool.
In another version, the displacement of the first drive member and the displacement of the second drive member may be obtained with the use of two separate motors coupled together electronically. The speeds of rotation of the first drive member and second drive member are therefore linked, so as to advance the spindle by a constant amount for each revolution.
Although such machines have proved generally satisfactory, their use for drilling bores (passages) in elements that consist of a plurality of layers of different materials can be tricky. For each of the materials involved there is both an optimum speed of rotation for the cutting tool (tool bit) and an optimum amount of advance per revolution.
If the drilling machine can be adjusted accordingly in order to satisfy the optimum conditions for drilling one of the layers, then drilling of the other layers takes place under conditions that are not optimized, and this results in bores of degraded quality and/or long drilling times.